Re: Heatsink for DPAK


"Pasquale" <desmomito@xxxxxxxxxxxx> wrote in message
news:483e80dd$0$29969$5fc30a8@xxxxxxxxxxxxxxxxxx
On Wed, 28 May 2008 17:02:29 -0700, David L. Jones wrote:

On May 29, 7:27 am, Pasquale <desmom...@xxxxxxxxxxxx> wrote:
No pictures handy, but: just put a plated-through hole underneath the
thermal pad in a rectangle of copper. To solder the device to the
plated through hole, turn the board over, stick the soldering iron
through the hole to heat up the device and the board, when it gets
hot enough to melt solder all around, stick in the solder and flood
the hole. Solder will wick between the device and the copper pad.

I used this for a low-scale production AD9954 DDS a few years back.

Having the board pretinned helps a bit if you aren't used to it.

Having the hole of a good size for your soldering iron tip really
helps. If not, you won't directly heat the devices thermal pad, just
the board.

Tim.

Thank you very much. I'll try your method. What about the heat ? I'm
trying to do some calculation if I need a heatsink or not. How did you
manage that ? I've see that I should use the Rthjc, Rthja and so on but
maybe a bigger copper pad will work. What do you think ?

Thank you,
Pasquale.

First you need to know how much power your device is going to dissipate.
This value will depend on how it's used in your circuit. If you don't
know how to calculate this far then I suspect you may have more to worry
about than whether or not you can solder a heatsink!

Dave.

I know, more or less, how much power my device will dissipate. For the
MOSFET should be something like Pd = Rds_on*I^2. For this reason I'm
trying to understand how to choose the right heatsink,

The device should have a degC/W figure for dissipation into ambient (no
heatsink).
If that temperature rise is acceptable in your application then no extra
heatsink is needed. In practice because the device will be soldered to the
board, it will have some extar heatsink already. If that temperature rise is
not acceptable then you have to start looking at thermal resistance figures
of the package and heatsink.
Generally, you can get a ballpark figure by simply taking the heatsink
degC/W figure and ignoring the thermal resistance losses (junction to case)
in the package etc. The heatsink I quoteed is 25degC/W, not marvelous, but
it helps.

Often with DPAK packages, a PCB heatsink is enough, and there are various
ways to improve this with layout. But PCB heatsinks can be harder to
calculate a figure for.

Dave.


.

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